Isochronous data can generally be described as data which is non-packetized and of indeterminate, potentially continuous duration. Examples of isochronous data sources include video cameras, which output a substantially continuous stream of data representing images and associated sounds, and telephones, which output a substantially continuous stream of voice data. An example of an isochronous data sink is a video monitor which receives a substantially continuous stream of video data for display.
FIG. 1A schematically depicts isochronous data transfer. The data transfer is first initiated, such as by initiating a telephone conversation or beginning a video camera transmission 12. After initiating the data transfer, transmission of the data is provided substantially continuously for an indeterminate period, until termination of the telephone conversation or video transmission 14. Every bit transferred need not necessarily represent a data bit. "Housekeeping" bits to control destination and timing may be also transferred. Furthermore, the data being transferred may comprise "Null" data such as silence during a telephone conversation or transfer of a blank video image. One type of isochronous data transfer is the Fiber Distributed Data Interface-II (FDDI-II) as described, for example, in FDDI-II Hybrid Multiplexer, Revision 2.4, dated Mar. 25, 1991.
The increasing availability of multi-media computers and work stations that include isochronous data sources and sinks in addition to non-isochronous sources and sinks has increased interest in the transfer of isochronous data in a network environment.
In a network environment, the receivers at both ends of the associated network link use a cycle template to enable the exchange of isochronous data. The cycle template continues to be exchanged even when the template contains no data and no isochronous communications are in progress. This continuous transmittal of data in the isochronous data path, requires the continual expenditure of network transmitter and receiver power. Power consumption directly influences the cost of operating the device and is of particular importance if the network component is battery driven. For example, if the network device is a lap top computer, the useful operating life of the device is a direct function of battery life.